Organic fluorescent probes are useful labeling for biomolecules. For in vivo applications, molecular imaging reagent is required to be biocompatible and to emit optical signals in the near infrared (NIR) region (700-900 nm), as NIR light can penetrate more deeply into biological tissues. On the practical side, optical imaging is dependent on the availability of the luminescent NIR reagents that exhibit high quantum yield, chemical and optical stability, and suitable pharmacological properties including aqueous solubility, specific binding, and low toxicity. At present time, most of the NIR probes are based on cyanine dyes, whose emission maxima are in the region of 650-900 nm. A notable drawback for the parent cyanine dyes is their small Stokes shift (typically about 20-50 nm), which hampers their broad application.
Among the new emerging design principles applied in fluorescent sensing, excited-stated intramolecular proton transfer (ESIPT) has recently received considerable attention due to its unique photophysical properties. Different from other organic chromophores, ESIPT molecules exhibit dual emissions from both the excited enol and keto tautomers, which are well separated from each other. In addition, emission of ESIPT dyes generally have large Stokes shift (ca. 150-200 nm), making them the ideal candidates for fluorescent sensors. Some ESIPT-based molecules, including 2-(2′-hydroxyphenyl)benzoxazole (HBO) and 2-(2′-hydroxyphenyl)benzimidazole (HBI), have been reported for cations and anion sensing. Most studies utilize ESIPT turn-off mechanisms since the interaction with a cation (or anion) removes the phenolic proton, thereby inhibiting ESIPT and resulting in blue-shifted fluorescence. Removal of the phenolic proton during metal chelation, however, permanently turns-off ESIPT. Thus far, only a few examples are known to utilize ESIPT turn-on mechanism in the chemosensor design, which involves the deprotection of the protected hydroxyl group. Among the known examples, nearly all ESIPT-based probes give emission in the visible region (400-650 nm).
As the second most abundant transition-metal ion in the human body, the Zn2+ ion is a component of enzymes and proteins, and plays an important role in various biological processes. In order to discover the vital roles of Zn2+ in biological processes, there is growing demand for sensing Zn2+ in living systems. Although many fluorescent chemosensors for Zn2+ cation have been studied, few near-infrared (NIR) fluorescent zinc probes are available to give emission in the desired 700-900 nm range. An ideal Zn2+ probe requires not only NIR emission (to minimize autofluorescence) but also large Stokes shift (for improved signal detection). It is thus desirable to incorporate the ESIPT process into the sensing scheme. Achieving the ESIPT emission signals in the NIR region, however, remains an attractive and challenging task.